This invention relates to an entirely digital tone and chord generator for an electronic musical instrument.
Prior electronic organs have used four bit binary words to represent the twelve notes in an octave. A binary-to-digital decoder enables a single note line in response to the presence of a particular binary word. The single note line then enables a gate in order to pass a cyclical tone signal from a separate tone generator to the voicing circuits. An example of such a circuit is shown in Tripp U.S. Pat. No. 3,544,693.
Tone generators have used both digital and analog circuitry to generate the plurality of cyclical tone signals which are gated under control of the digital note selector. A plurality of separate oscillators may be used for each note, or a single oscillator may be coupled to dividers interconnected to produce separate pulse trains representing the cyclical tone signals. In Freeman U.S. Pat. No. 3,236,931, for example, a voltage controlled oscillator generates a root note which is coupled to variable counters to produce a plurality of chordally related audible signals. Manually controllable keyboard bars actuate switches which alter the division ratios of some of the counters in order to produce submultiples of the root frequency which differ from each other by less than one octave. While use of variable ratio counters simplifies the tone generating circuit, the root note still must be generated by a controllable oscillator.
Since note selection has been considered separate from the audible tone generating circuits, separate tone selecting circuitry and tone generating circuitry has evolved in prior electronic organs. While tone selection has been digital in nature, and tone generation has been digital in nature, separate circuits have been required for these functions.